The present invention relates to the technical field of nanotechnology.
More particularly, the present invention relates to a process for producing metal nanoparticles and to the metal nanoparticles obtainable in this way and to the use thereof. The present invention further relates to dispersions comprising the inventive metal nanoparticles. The present invention finally relates to coating materials and coating systems, glasses and vitreous coatings, inks including printing inks, plastics, foams, cosmetics, cleaning compositions and impregnating materials, adhesives, sealing compounds and catalyst systems which comprise the inventive metal nanoparticles or the inventive dispersions.
There are numerous descriptions of syntheses for production of metal nanoparticles both in the scientific literature and in the patent literature. In most cases, production is effected via the reduction of an appropriate metal salt.
For example, such metal nanoparticles can be produced via the reduction of a metal salt (for example of a silver salt) in a biphasic reaction with sodium borohydride as a reducing agent. This involves first transferring the metal salt using tetraoctylammonium bromide, for example, from the aqueous phase to the organic phase (e.g. toluene or chloroform) and then reducing it by means of sodium borohydride. According to the use of a stabilizer, for example dodecanethiol, it is possible to synthesize virtually monodisperse metal nanoparticles and, on the basis of surface modification, disperse them in various media. For use in water, a phase transfer catalyst is required in most cases, for example 4-dimethylaminopyridine. One disadvantage in the case of this reaction regime is the lack of extendability to the industrial scale (“upscaleability”). In addition, the metal nanoparticles produced in this way cannot be modified for polar systems. A further disadvantage of this method is the use of relatively expensive starting chemicals, the varying yields and the multitude of by-products formed, especially the high coarse component of particles.
One alternative is that of reactions in which the reduction likewise takes place in an aqueous medium. These involve reducing a metal salt, especially a gold salt, for example by means of sodium citrate (called the “citrate method” according to Turkevich; cf., for example, Discuss. Faraday Soc. 11 (1951), 55). The disadvantage of this method is that the metal or gold concentrations achievable during the synthesis and in the later sol are very low. In addition, the metal nanoparticles obtained in this way can be isolated as a powder only in a very complex manner at best, if at all. Equally disadvantageous are the relatively high temperatures.
A further alternative is called the polyol method (on this subject, cf., for example, US 2006/0090599 A1), this method involving performing a reduction of a metal ion source in or by means of a polyol at elevated temperatures above 100° C., generally above 150° C. The polyol serves simultaneously as a stabilizer and solvent, i.e. no additional solvent is required. However, a disadvantage of this method is that the metal nanoparticles obtained can be isolated as such only in a very complex manner at best, if at all. In addition, the metal, nanoparticles obtained can be modified for nonpolar systems only with difficulty, if at all. A further disadvantage is the use of relatively expensive starting chemicals and the relatively high process temperatures.
In addition, the production of metal nanoparticles, especially gold nanoparticles, is also possible in principle by what is called sonolysis, but generally only on the experimental scale. This process is based on energy input by means of ultrasound. This involves reacting an aqueous solution, for example of HAuCl4, with glucose, the actual reducing agents being hydroxyl radicals and sugar pyrolysis radicals, which form at the interface region between the collapsing cavities of the glucose and the water. This results in what are called nanoribbons with widths of 30 to 50 nm and lengths of a few micrometers, these ribbons being flexible and being bendable to an extent of more than 90°. When glucose is replaced by cyclodextrin, a glycose oligomer, spherical gold nanoparticles are obtained. This method is relatively complex and cannot be applied to the industrial scale. In addition, relatively costly starting chemicals are used. Furthermore, this process can be performed, only with difficulty.
JP 2003-147418 A relates to the production of metal nanoparticles (e.g. Au or Pd) by reduction in micelles in aqueous media, the micelles being produced from amphiphilic block copolymers. The block copolymers needed for micelle formation are relatively complex to prepare and at the same time function as reducing agents.
US 2006/0266156 A1 relates to metal particles which comprise, on their surface, two different wetting agents or dispersants with different evaporation temperature, and a process for production thereof.
US 2006/0266157 A1 describes the production of metal nanoparticles by reduction of aqueous metal salt solutions in the presence of a wetting agent, for example cetyltrimethylammonium bromide (CTAB). The particles obtained in this way can be dispersed with addition of wetting agents or dispersants and combined with binders for coatings. The production is not effected in a purely aqueous medium. The reaction is a combination of citrate method on the one hand and biphasic reaction on the other hand. The coverage of the particle surfaces, for example with CTAB as a wetting agent, gives particles with good dispersibility in nonpolar media, but CTAB is relatively expensive and has to be used in a distinct excess. Furthermore, the addition of further dispersants is required in order to achieve a certain coating compatibility at all.
WO 2006/053225 A2 relates to the preparation of metal nanoparticle/protein complexes. The preparation is effected in an aqueous medium in the presence of proteins, such as BSA (bovine serum albumin), by reduction with NaBH4. Polyvinylpyrrolidone-coated silver particles are also described; in this case, the synthesis is effected in glycerol by what is called the polyol method.
WO 2006/072959 A1 relates to aqueous-based dispersions of metal nanoparticles and to a process for production thereof in the presence of a reducing water-soluble polymer which enables metal formation to form metal cores.
US 2007/0034052 A1 and US 2006/0159603 A1 describe the production of metal nanoparticles, especially silver nanoparticles, by reduction of metal ions by means of polyols.
U.S. Pat. No. 6,992,039 B2 relates to the preparation of supported, monodisperse noble metal nanoparticles on oxidic substrates. More particularly, the in situ production of noble metal nanoparticles on porous ceramics is described. The noble metal salts are reduced in the presence of metal alkoxides and wetting agents, followed by a subsequent calcining step.
US 2003/0199653 A1 relates to the production of metal nanoparticles in an aqueous medium in the presence of sulfur-containing copolymers by reduction with NaBH4. Owing to the use of sulfur-containing stabilizers, the particles obtained in this way are not usable for catalysis. Moreover, the synthesis is relatively complex. The re dispersibility of the particles obtained in this way is also not very great.
WO 02/087749 A1, CA 2 445 877 A1 and US 2004/0147618 A1 describe the production of silver nanoparticles in various media using gamma radiation or ultrasound in the presence of polymeric stabilizers.